In industrial scale reactions for hydrocarbon to olefin conversion, the desired products are typically lighter olefins such as ethylene and propylene. During these industrial processes, it is desirable to achieve nearly complete conversion of the oxygenate feedstock to olefins, such as the light olefins ethylene and propylene. In addition to controlling the overall conversion rate and the light olefin selectivity, it can also be desirable to control the ratio of ethylene to propylene produced by a reaction. A further consideration in conversion reactions is the cost required to perform the conversion reaction. A conventional reactor for performing a controlled oxygenates-to-olefin reaction can require multiple risers of nearly 200 feet in height and hundreds of tons of catalyst for converting the oxygenate feedstock.
U.S. Pat. No. 6,166,282 B1 discloses a process for converting oxygenates to olefins in a reactor with two reaction zones. Feedstock is introduced into a lower, dense phase zone from a single inlet to effect a partial conversion to light olefins. The dense phase zone includes a fluidized catalyst bed. It is disclosed that as the feedstock continues to move up in the reactor, it moves into a transition zone above the dense phase zone where the reaction continues essentially to completion. The dense phase zone operates at a superficial velocity of less than 1 meter per second, and the transition zone operates at a superficial velocity of from 1 to 4 meters per second.
United States Patent Application Publication No. 2004/0024276 describes a process for converting oxygenates to olefins in a reactor with a lower reaction zone and an upper reaction zone. Feedstock is introduced into the lower reaction zone, which includes a fluidized catalyst bed. It is disclosed that as the feedstock continues to move up in the reactor, the feedstock moves into the upper reaction zone having a smaller average diameter than the lower reaction zone. The upper reaction zone has a ratio of height versus average diameter of at least 5 to 1 but not greater than 100 to 1. The feedstock is further reacted in this second reaction zone. The lower reaction zone operates at a superficial velocity of at least 0.5 meters per second and up to about 10 meters per second, while the second reaction zone operates at a superficial velocity of at least 1 meter per second and up to about 25 meters per second.
U.S. Pat. No. 4,251,484 discloses a process for converting methanol and other lower alcohols and ethers to gasoline-type hydrocarbons. Baffle tubes, possibly including heat exchanger tubes, are placed in the reactor within a region having a fluidized bed of a zeolite catalyst. The disclosed range of catalyst bed densities is between 27 lbs/ft3 (432.5 kg/m3) and 35 lbs/ft3 (560.6 kg/m3), with a nominal gas velocity of about 2 ft/sec (0.61 m/sec) for the methanol passing through the catalyst bed. During a conversion reaction, the temperature of the catalyst bed is no greater than 775° F. After conversion, catalyst is separated from the product gas flow using conventional cyclone separators.
What is needed is a system and method for converting oxygenates to olefins that provides improved control over the process conditions. The process should allow for control over the ratio of ethylene and propylene produced. The process should also be able to be performed in a reactor having reduced dimensions and/or a lower catalyst requirement in the reactor.